The applicant proposes to investigate mechanisms of action of psychotropic drugs making use of mouse neuroblastoma cultures with a high catecholamine synthetic capacity. In developing this system, attention will be first focused on mechanisms regulating catecholamine biosynthesis, such as feedback inhibition by norepinephrine. Using subclones and cell hybrids of mouse neuroblastoma, the genetics of catecholamine biosynthesis will be studied. In addition tyrosine hydroxylase will be partially purified from these cells and its characteristics in the presence of the natural co-factor will be compared with those of the normal neuronal enzyme. The subcellular localization of tyrosine hydroxylase will be assessed and the possible formation of synaptosome-like particles upon homogenization will be examined.